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Abstract

"The purpose of this investigation was to determine the influence of the rate of loading variable on the apparent shear strengths of joints bonded with some of the currently available metal-to-metal adhesives. The effects of this variable are sometimes neglected by testing laboratories. The results of an experimental evaluation of the mechanical behavior of a group on modern metal-to-metal adhesives as a function of the rate of load application are used to more accurately establish the importance of rate of load considerations in adhesive joint testing. An attempt is made to explain observations of other investigators engaged in adhesive joint testing in terms of information gained from these experiments. The author has been unable to find published evidence of previous experimentation in this specific area of adhesive joint testing.

A series of tests was conducted by H. W. Eickner to determine the shear, fatigue, bend, impact and long-time-load strength properties of structural metal-to-metal adhesives. These tests were sponsored by the Air Force-Navy-Civil Subcommittee on Aircraft Design Criteria. While discussing a particular standard shear test, Mr. Eickner makes the following observation: "It is known that faster rates of loading will give somewhat higher failing loads in this test*, and some of the high shear values reported by other laboratories for some of these same adhesives were obtained when faster rates of loading were used than prescribed in the specification. It would seem that a rate of 600 to 700 pounds per minute on the 1/2-square-inch specimen would be reasonable, but some further tests should be made to determine the relationship between results with the two rates of loading."

U. F. Hribar of the Technical Staff of Hughes Aircraft Company reports the following observation: "Load rates for specimens tested under radiant heating conditions were four times as rapid as for hot dry air (approaching 5,000 psi per minute). Load rates up to 1,000,000 psi per minute have been found to have no appreciable effect on the tensile shear properties of high modulus* adhesives."

A test was designed to isolate the rate of loading variable and to measure its effects on the shear strength of some commercial adhesives. The results of this test are used to offer an explanation of the rate of loading effects observed in standard adhesive tests.

Some of the limitations of the present testing methods are also discussed. A brief review of the history of the use of adhesives is included to further establish the place of structural adhesives in present day technology and to illustrate the need for more complete and accurate testing methods.

* The test referred to is a lap-joint shear test"--Introduction, pages 1-2.